Recently, in the domain of ultra LSI silicon semiconductor devices, with the increase of integration density of semiconductor elements such as transistors and the reduction of chip size, geometric dimensions of the integrated semiconductor elements and internal interconnections of integrated circuits for electrically connecting those elements are consistently being miniaturized. The interconnection used in the silicon semiconductor devices is not limited to planar and two-dimensional interconnection. For example, in integrated circuits such as an ultra LSI, three-dimensional interconnections are utilized. Moreover, a damascene type structure (refer to the following non-patent document 1) using buried interconnections is also utilized. A contact plug is the one that is used for electrically connecting these interconnections and the transistor elements constituting the integrated circuits.
A contact plug formed in a contact hole (an opening to form the contact plug) that constitutes a part of an interconnection for silicon semiconductor integrated circuits is used for providing electric signals through an electrode made of metal silicide, etc., that is provided in an active region which realizes functions of a transistor constituting the integrated circuits. Here, the active region is a region where impurities are added in order to exert a function of a transistor element or an ion-implanted region. A source or a drain region is the example of the active region of a silicon transistor. A source or a drain region made of an amorphous silicon (a-Si) is also the example of the active region of a thin film transistor (TFT). At the active region, a transistor is generally formed with an electrode including a metal silicide layer (refer to “THIN FILMS-INTERDIFFUSION AND REACTIONS” by J. A. POATE, K. N. TU, and J. W. MAYER (Edi.) (John Wiley & Sons, Inc., USA, 1978 (ISBN 0-471-02238-1), pp 360-363)).
In a conventional technology, there is an example (refer to Japanese patent laid-open publication No. 2008-117853) where the contact plug of every electrode of the source, drain and gate of the transistor is made of tungsten (element symbol: W). The contact plug composed from tungsten (W) is electrically connected to a low-electrical-resistance interconnection made of aluminum (element symbol: Al) or copper (Cu).
Corresponding to the refinement of the contact hole based on high integration of the present silicon semiconductor integrated circuits, the contact plug that is provided at the active region made of silicon or the electrode made of metal silicide becomes even more refined. As a result, the present condition is that increase in electrical resistance of contact plug cannot be ignored, which adversely affects the signal delay (RC delay), and thus it has led to a problem in high speed processing of electrical signals. Therefore, a technology is proposed where the contact plug is formed using copper (Cu), which has lower specific resistance than tungsten (W) (refer to Japanese patent laid-open publication No. 2008-117853 and Japanese patent laid-open publication No. 2009-010037, and also refer to R. Islam et al., VLSI Proc., pp. 22-23 (2000), M. Inohara et al. IEDM Proc., pp. 4.6. 1-3 (2001), S. Demuynck et al., IITC Proc., pp. 178-179 (2006). Incidentally, it is said that the specific resistance of pure copper is 1.66 micro-ohm centimeter (μΩ·cm) (refer to Japanese patent laid-open publication No. H01-202841).
Japanese patent laid-open publication No. 2008-117853 discloses a technology where a contact plug that is composed of a layered structure of a tungsten (W) layer and a copper (Cu) layer on the tungsten (W) layer is configured at a source and a drain region of a transistor. In this layered structure, it is described that tungsten (W) fulfills a function as a so-called diffusion barrier layer, which deters atom diffusion of copper (Cu) into the source or drain region. Moreover, it is described that if the film thickness of the tungsten (W) that constitutes the layered structure is set to be one third of the depth of the contact hole at which the contact plug is provided or around 50 nanometers (Unit: nm), it can contribute to reduction of the electrical resistance of the contact plug.
However, even if the contact plug is composed of the layered structure of tungsten (W) and copper (Cu) following the conventional technology, it does not sufficiently stably lead to a contact plug with low contact resistance because tungsten (W) with large specific resistance exists internally in the contact plug. Moreover, it has not sufficiently stably achieved a plug with low electrical resistance as compared to the case of a contact plug composed completely of copper (Cu). Therefore, the current situation is that sufficiently stable production of system LSI that requires a transistor with low RC delay (refer to pages 345-346 of “Semiconductor Device (2nd edition)—Physics and Technology” by S. M. Sze (Oct. 5, 2005, Sangyo Tosho, Co., Ltd., third impression of second edition, pp 345-346)) and with high speed operation as a compositional element has not been achieved. Further, in the current situation, it is true that the magnitude of electrical resistance of a contact plug proves to be an obstacle for the supply of silicon integrated circuits with low power consumption.
Besides, “THIN FILMS-INTERDIFFUSION AND REACTIONS” by J. A. POATE, K. N. TU, and J. W. MAYER (Edi.) (John Wiley & Sons, Inc., USA, 1978 (ISBN 0-471-02238-1), pp 360-363) discloses the technology where the contact plug consists only of copper (Cu). As a diffusion barrier layer to prevent atomic diffusion of copper (Cu) in the contact plug consisting only of copper (Cu), a double layer structure is used where the first layer (lower layer) is tantalum (element symbol: Ta) or titanium (element symbol: Ti) and the second layer (upper layer: layer that contacts the copper (Cu) of the plug) is tantalum nitride (TaN) or titanium nitride (TiN).
In this way, in a case where the contact plug is formed only of copper (Cu), if the diffusion barrier layer of the double layer structure of the conventional technology is used, it will cause further inconvenience. This is because, whereas it is advisable to use a thin diffusion barrier layer to obtain a contact plug with low contact resistance, there are technical difficulties in having a thin layer, because the diffusion barrier layer composition revealed by the conventional technology requires two layers. In addition, problems have been found in providing the diffusion barrier layer of the double structure with a uniform film thickness especially in the refined contact hole with a small aspect ratio (a ratio of the width of the opening against the depth of the contact hole) and especially over the entire surface of the metal silicide exposed in the deep bottom.
Incidentally, another problem in the case where the contact hole is constituted from copper (Cu) is that copper (Cu) shows high reactivity to silicon or metal silicide exposed at the bottom surface of the contact hole in which the contact plug is formed, and moreover, the diffusion rate of copper atoms is also high. Therefore, even if a very thin diffusion barrier layer is used, if diffusion of copper (Cu) atoms cannot sufficiently be prevented, it causes problems of deterioration of ohmic characteristics or schottky characteristics of an electrode that constitutes the transistor because of the reaction of the copper (Cu) and the metal silicide, etc., for example. Accordingly, in the case of manufacturing an integrated circuit by integrating transistors of low performance by high density, if there are defects in the transistor elements forming that integrated circuit, it is expected that there will be problems to obtain an integrated circuit with normal characteristics.
Moreover, for example, even if a contact plug made of copper (Cu) is formed on a very thin barrier layer that is insufficient to prevent atom diffusion of copper (Cu) on a metal silicide layer of a source region or a drain region of a transistor, there is a problem that drain current leak is easy to occur due to the penetrating copper (Cu) atoms. Moreover, for example, even if a contact plug made of copper (Cu) is formed on a very thin barrier layer that is insufficient to prevent atom diffusion of copper (Cu) on a metal silicide layer formed at a gate region, there are problems that a gate current leak occurs and getting normal pinch-off characteristics is difficult, and so on, due to the penetrating copper (Cu) atoms. Moreover, a technique where a copper interconnection is configured with a manganese oxide (MnO) layer as a diffusion barrier layer against copper is disclosed (refer to “Chemical Vapor Deposition of Mn and Mn Oxide and their Step Coverage and Diffusion Barrier Properties on Patterned Interconnect Structures” by Kenji Matsumoto, Koji Neishi, Hitoshi Itoh, Hiroshi Sato, Shigetoshi Hosaka, and Junichi Koike (Applied Physics Express, Volume 2, 2009, published by the Japan Society of Applied Physics, Paper no. 036503, pp. 036513-1 to 036503-2) “Formation of manganese oxide barrier layer with thermal chemical deposition for advanced large-scale integrated interconnect structure” by K. Neishi, S. Aki, K. Matsumoto, H. Sato, H. Itoh, S. Hosaka, and J. Koike (Applied Physics Letters, Volume 93, 2008, published by American Institute of Physics, USA, Paper no. 032106)).
This invention is developed to overcome the above mentioned problems of the conventional technology. An objective of this invention is to provide a contact plug, a semiconductor device and a method for forming the contact plug. Here, the contact plug is able to stably suppress the diffusion of copper (Cu) atoms to a metal silicide film. Further, the contact plug is made from copper (Cu), which has low specific resistance that leads to low contact resistance.